Magnetron sputtering of rotating targets is well known and is used extensively for producing a wide variety of thin films on a wide variety of substrates. In the most basic form of rotating-target magnetron sputtering, the material to be sputtered from the target is either formed in the shape of a tube or is adhered to the outer surface of a support tube made of a rigid material. A magnetron assembly is disposed within the target and supplies a magnetic field, which permeates the target such that there is adequate magnetic flux at the outer surface of the target. The magnetic field is designed in a way such that it retains electrons emitted from the target so as to increase the probability that the electrons will have ionizing collisions with a working gas, hence enhancing the efficiency of the sputtering process.
A common class of materials deposited by magnetron sputtering is transparent conductive oxides (TCOs). The most commonly used TCO is indium tin oxide (ITO). ITO can be sputtered reactively, wherein a metal target is sputtered in an oxidizing atmosphere. However, sputtering ITO from a ceramic target is frequently preferred because its process is much easier to control. This results in more consistent high-quality films, compared with reactive sputtering. Planar targets have historically been preferred for ITO sputtering due to the lower fabrication costs compared to rotary ceramic targets.
In recent years, there has been a trend away from planar ITO target and towards rotary targets. This trend is a result of improved fabrication techniques that lower the cost of rotary ceramic targets, improved magnetron designs that increase the efficiency of material use, and the rotary cathode's superior operating performance, which helps satisfy increasing market demand for high quality films. Additionally, rotary targets hold a larger useable quantity of the sputtering material. This allows for reduced system maintenance requirements and, hence, better machine efficiency.
A sputtering method involving simultaneous application of direct current (DC) (or pulsed DC) power and radio frequency (RF) power has recently been applied to ITO targets. It has been demonstrated that improvement in the ITO film qualities resulted from the superposition of the two power sources using a planar target. The frequency of the power source used was 13.56 MHz, which is the most common frequency used for RF power in sputtering operations.
In another approach to applying RF power to a rotary cathode, RF energy is imparted into a target by transmitting an RF power signal via an antenna located within the rotating target. Here, the antenna is static and thus does not require a power transfer apparatus to move the power from static to rotating elements. This approach also disallows the superposition of DC power onto the RF driven target.
Recently, it has been shown that the application of RF power to a rotary cathode results in an unacceptable non-uniformity of the deposited film thickness. The non-uniformity is thought to be due to power (or voltage) attenuation, along the length of the cathode, resulting from the reactive impedance of the high-frequency of the applied voltage.
Accordingly, there remains a need for the capability of the application of RF power to rotary sputtering cathodes that addresses the issue of non-uniformity of deposited films.